1. Field of the Invention
The present invention relates to an optical switch. In particular, the invention relates to an optical switch expanding method which enables reduction in optical loss as well as to an optical switch formed based on such an expanding method. The invention also relates to an optical crossconnecting apparatus having such an optical switch.
2. Description of the Related Art
In recent years, multimedia communications as the Internet have spread rapidly. In the field of communications technologies, to cope with sharp increase in traffic due to such rapid spread of multimedia communications, intensive studies and developments have been made of optical communication technologies that enable ultra-long distance communication and large capacity communication. To accommodate further increase in traffic, there has been studied increasing the speed of the time-division multiplexing (TDM) transmission and the degree of multiplexing of the wavelength-division multiplexing (WDM) transmission. In optical crossconnecting apparatus, it is necessary to increase the numbers of inputs and outputs accordingly. It is desired to provide a proper method for expanding optical switches that are the core device of optical crossconnecting apparatus.
An optical crossconnecting apparatus accommodates a plurality of input and output optical transmission lines and routes, on a wavelength basis, a WDM optical signal input to an input optical transmission line, to desired output optical transmission lines. Since such routing is performed by an optical switch, expansion of the input/output ports in the optical switch is required for expanding (increasing the numbers of) the input/output ports in an optical crossconnecting apparatus.
FIGS. 13A and 13B are for explaining a conventional optical switch expanding method. FIG. 13A shows a 4xc3x974 optical matrix switch before expansion and FIG. 13B shows an 8xc3x978 optical matrix switch after expansion.
As shown in FIGS. 13A and 13B, the 4xc3x974 optical matrix switch 1001 is configured in such a manner that sixteen 2xc3x972 optical switch elements 1002 are arranged in a 4-row/4-column matrix. Such an nxc3x97n optical switch (n: integer) in a matrix will particularly be called an nxc3x97n optical matrix switch and a 2xc3x972 optical switch that is a minimum unit of the nxc3x97n optical matrix switch will be called a 2xc3x972 optical switch element.
Conventionally, in expanding such a 4-input/4-output 4xc3x974 optical matrix switch 1001-1 to an 8-input/8-output 8xc3x978 optical matrix switch 1011 in terms of the input/output ports, three optical matrix switches 1001-2 to 1001-4 are provided additionally, the four optical matrix switches 1001-1 to 1001-4 are arranged in a matrix, and the input ports and the output ports in two of the optical matrix switches 1001-1 to 1001-4 that are adjacent to each other vertically or horizontally are connected to each other.
The optical switch elements 1002 of the 8xc3x978 optical matrix switch 1011 are assigned row numbers in order of the first input port to the eighth input port and assigned column numbers in order of geometrical closeness to the input ports. The row numbers and the column numbers assigned are given to the optical switch elements 1002 as suffixes each being an array of a row number and a column number that are arranged in this order. For example, in FIG. 13B, the optical switch element that is connected to the second input port and located fourth as counted from the input port is the second-row/fourth-column optical switch element and hence is given a reference symbol 1002-24. The optical switch element that is connected to the sixth input port and located eighth as counted from the input port is the sixth-row/eighth-column optical switch element and hence is given a reference symbol 1002-68. To avoid unduly complicating FIG. 13B, only part of the reference symbols of the optical switch elements 1002 are drawn in the figure.
For the sixty-four optical switch elements 1002, control symbols to be used for a control of routing an optical signal that is input to the optical matrix switch 1011 to a desired output port are assigned in the following manner. They are assigned so as to specify, by using an input port position and an output port position, an optical switch element 1002 where switching should be made for routing to a desired output port. In FIG. 13B, each of such control symbols is an array of S, an input port number, and an output port number that are arranged in this order. For example, the optical switch element 1002-11 is given a symbol S11. An optical signal that is input to the first input port can be routed to the first output port by switching at the optical switch element 1002-11 (S11). The optical switch element 1002-75 is given a symbol S75. An optical signal that is input to the seventh input port can be routed to the fifth output port by switching at the optical switch element 1002-75 (S75).
In the optical matrix switch 1011 obtained by expanding the 4-input/4-output optical matrix switch 1001 in the above method, an optical signal passes through fifteen optical switch elements at maximum, in which optical loss is large. For example, to output, from the eighth output port, an optical signal that is input to the first input port, switching is performed at the optical switch element 1002-18 (S18). Therefore, the optical signal passes through the fifteen optical switch elements 1002-11, 1002-12, 1002-13, 1002-14, 1002-15, 1002-16, 1002-17, 1002-18, 1002-28, 1002-38, 1002-44, 1002-58, 1002-68, 1002-78, and 1002-88. Losses in those optical switch elements 1002 sum up to a large loss.
On the other hand, an optical that is input to the eighth input port can be routed to the first output port by switching only at the optical switch element 1002-81 (S81). This optical signal passes through only one optical switch element 1002-81.
As a result, a difference approximately corresponding to the losses in 14 optical switch elements 1002 occurs between the optical output level of the optical signal that has passed through the one optical switch element 1002 and that of the optical signal that has passed through the 15 optical switch elements 1002.
Incidentally, since an optical signal that is output from an optical matrix switch is input to an optical component such as a photodetector of an optical receiver, its optical output level should be higher than a certain level. However, loss occurs in each optical switch element. Therefore, where routing is performed by an optical matrix switch, the maximum number of optical switch elements through which an optical signal passes determines a switch size (i.e., the numbers of inputs and outputs) of the optical matrix switch. Therefore, the conventional expanding method and optical matrix switches according to the conventional expanding method have a problem that the matrix optical switch cannot be large in size because as the degree of expansion increases, the maximum number of optical switch elements through which an optical signal passes increases and the loss rises accordingly.
Large differences between the output levels of the respective output ports in an optical matrix switch cause a problem that optical components connected to the output ports such as optical amplifiers or photodetectors should have a wide input dynamic range or plural kinds of optical components having different input dynamic ranges should be prepared.
An object of the present invention is therefore to provide an optical switch expanding method which enables expansion of an optical switch with a smaller loss than in the conventional art, as well as an optical switch in which connections are made according to the expanding method and an optical crossconnecting apparatus where the optical switch is employed.
Another object of the invention is to provide an optical switch expanding method which enables expansion of an optical switch with smaller differences between the levels of output light than in the conventional art, as well as an optical switch in which connections are made according to the expanding method and an optical crossconnecting apparatus where the optical switch is employed.
The invention provides an optical switch expanding method for increasing the number of inputs and outputs of an optical switch comprising first to fourth optical matrix switches in which a plurality of 2-input/2-output optical switch elements are arranged in a matrix to form a plurality of input ports, a plurality of auxiliary input ports, a plurality of output ports, and a plurality of auxiliary output ports.
The optical switch expanding method comprises the steps of: respectively connecting the auxiliary output ports in the first optical matrix switch to the input ports in the third optical matrix switch; respectively connecting the output ports in the second optical matrix switch to the auxiliary input ports in the third optical matrix switch; respectively connecting the output ports in the first optical matrix switch to the auxiliary input ports in the fourth optical matrix switch; and respectively connecting the auxiliary output ports in the second optical matrix switch to the input ports in the fourth optical matrix switch.
The invention also provides an optical switch comprising first to fourth optical matrix switches wherein a plurality of 2-input/2-output optical switch elements are arranged in a matrix to form a plurality of input ports, a plurality of auxiliary input ports, a plurality of output ports, and a plurality of auxiliary output ports. The auxiliary output ports in the first optical matrix switch are respectively connected to the input ports in the third optical matrix switch, the output ports in the second optical matrix switch are respectively connected to the auxiliary input ports in the third optical matrix switch, the output ports in the first optical matrix switch are respectively connected to the auxiliary input ports in the fourth optical matrix switch, and the auxiliary output ports in the second optical matrix switch are respectively connected to the input ports in the fourth optical matrix switch.
Each of the first to fourth optical matrix switches may be a Cross-bar optical matrix switch.
The 2-input/2-output optical switch elements may be semiconductor optical switches.
The 2-input/2-output optical switch elements may be optical switches in an optomicro-electromechanical system.
Each of the first to fourth optical matrix switches may be a PI-LOSS optical matrix switch.
The invention further provides an optical crossconnecting apparatus comprising: a plurality of optical demultiplexing sections for demultiplexing, on a wavelength basis, input light to be output from a plurality of output ports; a plurality of optical multiplexing sections for wavelength-multiplexing optical signals that are input to a plurality of input ports; and an optical switch comprising first to fourth optical matrix switches wherein a plurality of 2-input/2-output optical switch elements are arranged in a matrix to form a plurality of input ports, a plurality of auxiliary input ports, a plurality of output ports, and a plurality of auxiliary output ports. The auxiliary output ports in the first optical matrix switch are respectively connected to the input ports in the third optical matrix switch, the output ports in the second optical matrix switch are respectively connected to the auxiliary input ports in the third optical matrix switch, the output ports in the first optical matrix switch are respectively connected to the auxiliary input ports in the fourth optical matrix switch, and the auxiliary output ports in the second optical matrix switch are respectively connected to the input ports in the fourth optical matrix switch.
In the optical crossconnecting apparatus (optical matrix switch) according to the invention, it is possible to reduce optical loss and differences between the levels of output light of the respective output, compared to the conventional art. Therefore, the input dynamic range of optical components that are connected to the output ports in the optical crossconnecting apparatus (optical matrix switch) can be reduced.